In cellular wireless systems, both current systems such as, for example, UTRA (UMTS Terrestrial Radio Access) systems, and future systems such as the LTE (Long Term Evolution) systems, a principle which is used is so called TDD, Time Division Duplex, i.e. a principle according to which uplink and downlink traffic occur during different periods of time, so called subframes, which are comprised in a larger frame. Usually, in a TDD system, the uplink and downlink traffic use the same carrier frequency,
Due to the fact that uplink and downlink traffic share one and the same frequency in a TDD system, interference problems may occur between different cells in the system. In particular, downlink traffic from one cell may cause interference in other cells.
In addition to interference between different cells in one and the same TDD system, interference may also occur between different cells in different but co-located or adjoining systems, for example systems which are run by different operators.
One way of reducing the problems of inter-cell interference in TDD systems is to arrange so called “guard periods” at the transitions between downlink and uplink traffic, i.e. periods during which no traffic may occur. Guard periods may also be arranged at transitions from uplink to downlink.
Co-existence, i.e. the ability to avoid interference between cells in one and the same system, as well as between cells of adjoining or co-located cells of different systems, is an important factor.
However, the frame structures in some current UTRA and LTE TDD standards offer limited co-existence opportunities. To make efficient co-existence possible, the frame structure should preferably make it possible to have a large flexibility when it comes to configuring it to overcome interference problems, both interference from cells in the “own” system as well as from cells in co-located or adjoining cells of other systems.